Methods of preparing fire doors

ABSTRACT

Fiberboards are provided containing at least about 65 wt. % calcium sulfate dihydrate derived from hemihydrate or anhydrous gypsum, and at least about 7 wt. % pulped paper fibers. The boards are capable of being formed into structural members having superior mechanical properties and/or fire resistance, including screw-holding capacities in excess of 650 lbs.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.08/209,615, filed Mar. 10, 1994, (abandoned), which is a continuation ofU.S. application Ser. No. 07/937,361, filed Aug. 27, 1992 (abandoned),which is a continuation of application Ser. No. 07/699,676, filed May14, 1991 (U.S. Pat. No. 5,155,959), which in turn is a divisional ofU.S. application Ser. No. 07/420,362, filed Oct. 12, 1989 (U.S. Pat. No.5,171,366).

FIELD OF THE INVENTION

This invention is in the field of building materials, especiallyfire-resistant building materials which contain gypsum in combinationwith certain fibrous additives, including paper fiber, as well asmethods for producing such materials and to articles which incorporatethem, such as fire doors.

BACKGROUND OF THE INVENTION

Various products containing the naturally-occurring mineral, gypsum,have been developed for the building trades. Especially notable arevarious gypsum wall board products, e.g., sheet materials typicallyfaced with paper. Desirable characteristics of such products includetheir strength at relatively low density (about 0.7 gm/cm³), ease ofhandling and fabrication, and low cost. In producing gypsum buildingproducts, calcined gypsum, i.e., anhydrous or hemihydrate (CaSO₄.1/2H₂O), known also as stucco, plaster of paris, molding plaster, buildingplaster, and the like, typically in an aqueous slurry, is cast, molded,and dried. During the course of this process, the calcined gypsum isfurther hydrated, yielding CaSO₄.2H₂ O.

Although the hydration adds only about 20% to the weight of the calcinedgypsum, excess water generally is present in the slurry to decrease theviscosity and facilitate molding. However, the excess water isundesirable in other respects. For example, it must be removed insubsequent processing of the product, at considerable cost. Furthermore,the resultant dried product is of low density and compressive strength.Consequently, it is conventional wisdom in the art that the amount ofwater should be held to a minimum, and various additives and treatmentshave been proposed to fluidize the slurry, but minimize the water; e.g.,U.S. Pat. Nos. 2,913,308; 4,222,984; and 4,252,568. "Dry" or "semi-dry"processes have been described in which water required for the hydrationchemistry is supplied by a water-saturable filler, such as woodshavings, wood fiber granulate and bark; e.g., U.S. Pat. No. 4,328,178.In this regard, the article entitled "Inorganically Bonded Wood" by A.A. Moslemi, Chem. Tech., Aug. 1988, pp 504-510, summarizes the state ofthe art.

A number of building materials have been disclosed in which substantialquantities of cellulosic fillers, including wood particles and fibers,have been included in gypsum-containing products, not only as aids toincorporating the water necessary to hydrate the gypsum, but also tostrengthen and otherwise upgrade the mechanical properties of theproduct. For example, U.S. Pat. No. 3,951,735 discloses a strengthenedgypsum wallboard material having a density of 1.1-1.6 gm/cm³ (obtainedby compressing a lamination) which includes calcined gypsum andcellulosic fiber such as paper pulp, but the criticality of alsoincluding a substantial amount of asbestos fiber is pointed out.Similarly, U.S. Pat. No. 4,127,628 discloses a multi-layered gypsumproduct of low density (0.3-0.9 gm/cm³) which includes glass fibers andoptionally also contains pulp and polyvinyl alcohol, but, in addition, asubstantial amount of asbestos fiber. Products which require asbestos toattain superior properties are difficult to justify in view of theenvironmental and health hazards associated with such products.

U.S. Pat. No. 4,239,716 describes a gypsum-containing building productcontaining a reinforcing agent which may be wood pulp or glass fibers,together with a binder, such as polyvinyl acetate. However, thedisclosure is limited to the use of fibrous alpha-calcium sulfatehemihydrate, a very expensive raw material, requiring special conditionsto produce, which is difficult to reconcile if equivalent propertiescould be obtained in a product which employs common and inexpensivenon-fibrous forms of calcined gypsum.

Gypsum building materials generally are held in high regard for use infire-resistant construction. The spread of fire and the penetration offlame through set gypsum structures is delayed, because impinging heatinitially operates to reverse the hydration reaction, recalcining thegypsum, liberating water. The liberated water is an additional energysink, absorbing its heat of vaporization. Finally, however, although thegypsum doesn't burn, it shrinks and cracks when heated in a flame. It isknown that this tendency to crack can be countered with appropriateadditives, such as fiber, especially glass textile fibers, which holdthe structure together, and raw vermiculite, which expands when heated,counteracting the gypsum shrinkage. With this knowledge, a number ofgypsum-containing products have been developed in which fire-resistanceis critical. Such products include fire doors, for example.

Fire doors may be of either the panel or flush types. They includefacings on the two major surface, and the core of the door may either besolid or at least partially hollow. Edge banding is included around thedoor periphery for aesthetic or structural reasons.

Fire doors, as used in residential, commercial and industrialapplications, are typically employed in conjunction with fire walls toprovide fire protection between different zones of a structure, andparticularly to isolate high fire risk areas of a building from theremainder of the structure, such as the garage of a dwelling from itsliving quarters. Fire doors usually are not capable of indefinitelywithstanding the high temperature conditions of a fire but, rather, aredesigned to maintain the integrity of the fire wall for a limited timeto permit the occupants of a building to escape and to delay the spreadof the fire until fire control equipment can be brought to the scene.

Various tests have been devised for fire doors and are based on factors,such as the time that a given door would withstand a certain temperaturewhile maintaining its integrity, and hose stream tests which involve thedoor's ability to withstand the forces of a high pressure water stream.The American Society for Testing Materials (ASTM) has devised tests toestablish fire door standards, and these standards are incorporated intobuilding codes and architectural specifications. One such standard, ASTMMethod E 152, requires a door to maintain its integrity for periodsranging up to 1.5 hrs. while withstanding progressively highertemperatures and the erosive effects of a high pressure fire hose at theconclusion of the fire exposure.

Considerations in fire door design, in addition to retarding the advanceof a fire, include the cost of raw materials and the cost offabrication. Furthermore, the weight of the door is important, both fromthe standpoint of ease in handling and the cost of transportation. Thestrength of the door is also a significant factor, since fire doors mustpass the previously noted water stream tests as well as have therequisite structural strength to withstand normal use and abuse.

Fire-resistant doors have been made in a variety of constructionsutilizing a number of different materials, including wood, metal andmineral materials. Early forms of fire doors simply comprised woodencores faced with metal sheeting. Although wood of ample thickness is aneffective fire and heat retardant, doors of such construction tended tobe heavy and were expensive to fabricate and transport.

Mineral materials have also been employed in the manufacture of firedoors. The core of a commercial metal fire door principally comprises acomposition including mineral fibers and a binder. Such doors suffer,however, from a lack of strength, and handling the friable cores resultsin the production of irritating dust particles during the manufacturingprocess.

It has also been proposed to make fire doors wherein the core comprisesparticles of expanded perlite which are bound together by the use ofvarious hydraulic binders including gypsum, cement and inorganicadhesive material. In order to provide sufficient strength, particularlyto withstand handling of the core during manufacture, the core iscompressed to compact the mixture to a relatively high density,resulting in a heavy door.

Other fire doors have included conventional gypsum wallboard panels as acore material. However, in order to provide sufficient fire resistance,the thickness required of the wallboard is such as to result in anexcessively heavy door. Furthermore, internal structural members such asrails or mullions have been found necessary to support and strengthenwallboard panels. The need for such reinforcing elements increases thecost of materials and assembly of such doors. In addition to theabove-mentioned considerations, fire doors must, in order to becommercially acceptable, also have other properties that are related tothe manufacture, installation and service of the fire-resistant door.

U.S. Pat. No. 4,159,302 discloses a set gypsum-containing compositionwhich is especially useful as the core in a solid core fire door, andU.S. Pat. No. 4,811,538 describes a fire door which is partially hollowbut has a core of set gypsum faced with fibrous mats. U.S. Pat. No.4,748,771 discloses set gypsum-containing edge banding for use in firedoors.

The state of the art edge banding described in U.S. Pat. No. 4,748,771is of tripartite construction, in that it includes in lamination, aninner strip comprising a cast gypsum mixture, an intermediatefiber-reinforced plastic strip, and an outer strip of natural wood. Suchedge banding is surprisingly complex and correspondingly expensive. Thecomplexity is necessary, at least in part, because the combination ofthe gypsum and wood strips alone does not provide the screw-holdingcapacity required for hinges, latch mechanism, etc.; the thin plasticstrip is necessary solely for that reason. The gypsum strip included inthe edge banding includes gypsum, glass fiber, raw vermiculite, andclay, together with a small amount of paper fiber (less than 2% byweight), wood chips, and a resin binder, which may be polyvinyl acetate.

In summary, the available gypsum building materials which have thesuperior mechanical and fire-resistant properties required intechnically advanced products such as fire doors often require expensiveand potentially hazardous additives, such as asbestos, to achieve thoseproperties. Thus, it is an object of the present invention to providenovel gypsum compositions including safe and inexpensive components fromwhich superior building materials can be made. It is another objectiveof the invention to provide a process for making such superior buildingmaterials from the novel compositions. Yet another objective is theprovision of gypsum building materials having superior mechanical andfire-resistant properties. It is still another objective to providenovel fire doors which incorporate the novel building materials.

SUMMARY OF THE INVENTION

Consequently, this invention provides in one aspect a composition forpreparing fire-resistant structural building materials having density ofat least about 60 lbs./cu. ft., flexural strength, measured as definedhereinafter, of at least about 40 lb. (1/2 in. thick material), andscrew-holding capacity, measured as defined hereinafter, of at leastabout 400 lbs. The building material does not require facing, but, ifdesired, may be faced with any suitable material such as, for example,the paper facing commonly employed in gypsum wallboard, or glass matfacing, for example, as described in U.S. Pat. No. 4,810,569, or adecorative overlay. The composition from which the building material ismade comprises an aqueous dispersion of solids which includes by weightabout 53% to about 78% calcium sulfate, about 7% to about 30% cellulosicfiber, and preferably about 1.5% to about 35% performance boosterselected from inorganic fiber, clay, vermiculite and binder polymer,together with a quantity of water in excess of that required tocompletely hydrate the calcined gypsum.

In other aspects, this invention provides a process for manufacturing afire-resistant set gypsum structural building material which exhibitsthe aforesaid properties, as well as certain building materials per sewhich possess some or all of the aforesaid characteristics. Inadditional aspects, the invention provides fire doors which meetspecific fire resistance criteria. The invention provides a method ofpreparing a fire door capable of achieving a 20 minute ASTM E-152 firetest rating without the use of asbestos, comprising: (a) providing atleast a pair of substantially asbestos free preformed gypsum fiberboardpanels having a composition comprising about 65 wt. % to about 90 wt. %set gypsum dihydrate, about 10 wt % to about 17 wt. % paper fiber, and adensity of at least about 60 lbs/cu.ft., said fiberboard panels beingformed by casting and compressing a wet slurry comprising gypsum andsaid paper fiber; (b) providing a core; (c) adhering said core betweensaid gypsum fiberboard panels; (d) securing edge banding between saidgypsum fiberboard panels to substantially enclose said core and form acomposite member, said edge banding comprising about 65 wt. % to about90 wt. % set gypsum dihydrate and from about 7 wt. % to about 30 wt. %paper fiber, said edge banding having a density of at least about 60lbs/cu.ft. and a screw holding capacity of at least 400 pounds; and (e)providing said composite member as a substantial portion of said door soas to produce a fire door capable of achieving a 20 minute ASTM E-152fire test rating.

Another aspect of the present invention involves a method formanufacturing an asbestos-free structural building material from calciumsulfate and paper fiber in which the paper fiber is combined with thecalcium sulfate in the form of a pulp of paper stock that contains atleast about 20 times more water than paper stock.

There are many advantages associated with the practice of the presentinvention. In addition to providing practical and economical means forproducing a product which has excellent functional characteristics, theinvention can be practiced in a manner such that important and desirableenvironmental benefits can be realized. Thus, as explained hereinbelow,sources of essential constituents which comprise the product of thepresent invention can be scrap- or waste-like materials which in thisday and age are generally considered to be the cause of expensive wastedisposal problems. The present invention allows such materials to be putto good use.

This invention will be understood more completely by reference to thedrawings, which disclose preferred embodiments of the inventioncontaining certain optional features, and to the detailed descriptionwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram which describes the process of this inventionby which the building materials can be made.

FIG. 2 is a graphical illustration of the effect of one of thecomponents in a composition of this invention on one of the processingsteps.

FIG. 3 graphically illustrates the flexural strength of a typicalbuilding material of this invention as a function of the density of thematerial.

FIG. 4 graphically illustrates the screw-holding capacity of a typicalbuilding material of this invention as a function of the density of thematerial.

FIG. 5 is a top view of one of the fire doors within the scope of thisinvention.

FIG. 6 is a cross-sectional view taken along line 6--6 in FIG. 5.

FIG. 7 is a front elevational view of another fire door within the scopeof this invention.

FIG. 8 is an isometric view of a corner of the fire door of FIG. 7 withportions broken away.

DETAILED DESCRIPTION OF THE INVENTION

One of the essential constituents comprising the product of the presentinvention is calcium sulfate dihydrate. This constituent is derived fromthe hydration of any form of calcium sulfate which is in non-fibrousform and which is capable of reacting with water to form set gypsum,that is, calcium sulfate dihydrate. Thus, the calcium sulfate can be inanhydrous form or in the hemihydrate form. It is believed that thehemihydrate form of calcium sulfate will be used most widely. Of the Bforms of the hemihydrate, use of the latter is preferred. Thehemihydrate can be produced from the naturally-occurring gypsum mineralby heating, or calcining, the dihydrate.

For many applications, it is not important to inquire into thecrystalline form of the hemihydrate; however, with respect to thisinvention, that is not the case. It is known that calcium sulfatehemihydrate can exist in two different crystalline forms, namely anon-fibrous form and a fibrous form, for example, elongated needles,such as the fibrous alpha-calcium sulfate hemihydrate disclosed in U.S.Pat. No. 4,239,716. In the practice of this invention, there is used anon-fibrous form of calcium sulfate capable of reacting with water toform set gypsum. It should be understood, however, that a minor amountof a fibrous form of gypsum can be used as an optional constituent.

As mentioned above, one of the advantages of the present invention isthat waste-type materials can be used in the practice of the presentinvention. For example, there can be used as the source of the calciumsulfate the material known as "desulfurized by-product gypsum" which isproduced by the desulfurization of flue gas. Another example of a waste-or scrap-type material that can be used in the practice of the presentinvention is scrap gypsum wall board, which can be used as a source ofboth calcium sulfate and the paper constituent of the building product.For this purpose, scrap paper-faced gypsum wallboard can be ground intosuitably small particles which are calcined in water under pressure andin the presence of a crystal modifier to form non-fibrous calciumsulfate hemihydrate. Scrap gypsum wallboard can also be transformed intoa suitable material for use in the practice of the present invention bygrinding and calcining it at atmospheric pressure. Sufficient water canbe used to form the desired pulp-type material from which the product isconveniently made.

A mixture of the different sources of calcium sulfate can be used in thepractice of the present invention.

In the use of an aqueous dispersion to make the building product of thepresent invention, the nonfibrous calcium sulfate generally willcomprise between about 53% and about 78% by weight of the total solids,preferably between about 55% and about 70% by weight, depending upon thespecific application for the resultant building material. For example,in a building board to be employed as edge banding in a fire door, thenonfibrous calcium sulfate content of the dispersion may be in the rangeof about 54% to about 62% by weight of the solids. On the other hand, ina building panel to be used as the facing on a fire door, the calciumsulfate content of the dispersion may lie between about 60% and about80% by weight of the solids.

The gypsum dihydrate content of the building materials and products ofthis invention will be approximately 18.5% greater than the nonfibrouscalcium sulfate content of the compositions from which they are made,the difference representing the added water of hydration in the setgypsum dihydrate. That is, by weight, the set gypsum will comprisebetween about 65% and about 90%, preferably between about 70% and about85%. In a building board for use as edge banding in a fire door, the setgypsum dihydrate may comprise between about 68% and about 78% by weight.In a building panel intended for use in fire door facings, the setgypsum may comprise between about 75% and about 90% by weight.

The composition, process, building material and product aspects of thisinvention also employ substantial amounts of cellulosic fiber.Cellulosic fiber includes the fibrous component of plants, such ascotton, linen, and flax, for example. Among the various sources ofcellulosic fiber, paper stock is conveniently employed. That is, thesolid component involved in each of the aforesaid aspects of theinvention includes by weight about 7% to about 30% paper fiber,preferably between about 10% and about 17%. Building materials intendedfor use in various specific products may contain somewhat differentamounts of paper fiber. For example, a building board intended for useas edge banding in a fire door may contain about 15% paper fiber byweight, while a panel intended to be used as fire door facing maycontain somewhat less, e.g., about 12% or 13%. The presence of the paperfiber makes it possible to produce building materials having goodphysical characteristics such as flexural strength, screw holding andsurface hardness without having any separate surfacing membrane such asthe paper surfacing on conventional gypsum board.

The paper fiber can be derived from either virgin paper stock, orpreviously used, waste paper stock can be employed. The source of thepaper can be wood, cotton or linen rags, straw, etc., the origin orhistory of the paper not being important factors. The paper may be aproduct of the sulfite process, the sulfate (Kraft paper) process, orother processes. Among the types of paper stock that have beensuccessfully employed are virgin and brown Kraft papers and, especially,newsprint. Waste newspaper provides very satisfactory results, isinexpensive, and its use helps to overcome an environmental pollutionproblem. And, as mentioned hereinabove, the source of the paper stockcan include the paper of ground paper-faced gypsum wallboard.

Products within the scope of this invention desirably and preferablyinclude one or more performance boosting additives, their specificnature depending to some extent on the intended utility of the finalproduct. In almost every case, there will be desirably used one or moredefoaming agents, dispersants and accelerators, ingredients which arewell known in the art and are employed at low concentration levels,generally each at less than about 1% by weight of the solids. In theaggregate, the performance booster generally will comprise about 1.5% toabout 35% by weight of the solids and will preferably be selected frominorganic fiber, clay, vermiculite, and binder polymer.

Inorganic fiber, as that term is employed herein, includes glass textilefiber and mineral wool. These latter terms are defined in U.S. Pat. No.4,557,973, and those definitions are incorporated herein by reference.Briefly, the term "mineral wool" means glass or other mineral fibersprepared by attenuating a melt of glass, basalt, blast furnace slag orother vitreous mineral composition from the face of a heated centrifugalrotor or the like. This process is in contrast to that used to producetextile fibers, where the melt is drawn through an orifice. Anespecially useful and readily available type of mineral wool is glasswool as found in glass wool insulation material. Glass textile fiber andglass wool, jointly or severally, are referred to herein as "siliceousfiber." As employed in this invention, the glass textile fiber generallywill be chopped, e.g., the fibers may be about 1/2 inch long.

The composition, process, building materials and specific products ofthis invention preferably include siliceous fiber. Siliceous fiberimproves the fire resistance of the building materials and products ofthis invention, apparently by decreasing the tendency of the gypsumconstruction to crack under thermal stress. The siliceous fibercomprises up to about 7% by weight and may include glass textile fiberand, in addition, glass wool, depending upon the specific product. Forexample, a building board intended for use as edge banding in a firedoor preferably includes up to about 7% by weight glass textile fiber,most preferably about 2% by weight glass textile fiber, glass wool beingunnecessary. However, a building panel intended to be used as facing ona fire door preferably includes about 0.8% to about 2% siliceous fiber,most preferably a combination of about 0.4% chopped glass textile fiberand about 0.5% to about 1.5% glass wool by weight.

The performance booster may also include either clay or vermiculite, orboth, especially if the intended building material or product requiresexcellent fire resistance. Both of these materials may be present inamounts up to about 6%, preferably about 3-4% by weight of the solids.The clay to be employed will generally be kaolin clay, which iseffective to control the shrinkage of the gypsum-containing constructionunder extreme heat. The vermiculite should be raw, or unexpandedvermiculite, which swells when heated, helping to control shrinkage ofthe construction and possible cracking. The requirement for the presenceof these materials depends somewhat on the intended use for the finalproduct and may not be necessary in, e.g., a building panel intended foruse as facings in a fire door.

The composition of this invention, as well as the process for employingthat composition to make building materials and specific products withinthe scope of this invention, may also include binder polymer. The binderpolymer affects the physical properties of the building materials andproducts, especially their flexural strength, and also permits goodfastener retention at lower density. Furthermore, the binder polymerimproves the surface characteristics of the product, such as smoothingthe surface and making it easier to finish.

The binder polymer, when present, may comprise up to about 15% by weightof the solids, but preferably about 1% to about 3% by weight. A numberof different polymeric materials may be employed as binder polymer,including homopolymers, such as poly(vinyl acetate) and polyacrylate, aswell as copolymers, such as poly(ethylene)-co-(vinyl chloride),poly(styrene)-co-(butadiene), and poly(vinyl acetate)-co(methylacrylate). Among the various binder polymer possibilities, esters ofpoly(vinyl alcohol) are especially effective, and poly(vinyl acetate)homopolymer is preferred. It is also convenient in most cases tointroduce the binder polymer as an aqueous emulsion, many of which arecommercially available.

The composition of this invention will also include water in an amountin vast excess of that required to react with and hydrate the calcinednonfibrous gypsum. That is, preferably at least about a 25-fold excessamount of water should be present. Contrary to the conventional wisdom.,the excess water provides processing advantages and leads to productswhich possess superior properties.

Although the composition of this invention may be formulated in manydifferent ways, and any number of different techniques may be employedto produce the building materials of this invention, a process which ispreferred for making these materials is illustrated diagrammatically inFIG. 1. With reference now to FIG. 1, the paper fiber component, e.g.,newspaper, together with water, at least about 20 times as much water byweight as paper, are added to pulper 20, and the mixture is reduced topulp, producing a substantially homogeneous suspension. Glass wool, ifit is specified in the composition, can be separately pulped in at leastabout 20 times its weight of water and the separately pulped wool andpaper combined. Alternatively, the paper and glass wool can be pulpedtogether, if desired, in at least about 20 times their combined weightof water. Any textile glass fiber, clay and vermiculite called for arethen added to the pulper and thoroughly mixed and incorporated into thesuspension. The suspension is then transferred to tank 21.

As needed and required for the composition, pulped suspension from tank21 is added to mixer 24, any polymer binder from tank 22 as may berequired for the composition is added to mixer 24, and sufficientnonfibrous calcium sulfate to yield the requisite amount of dihydratewhen reacted with less than about 5% of the water is added from tank 23to mixer 24, wherein all the components are mixed and incorporated intothe suspension, producing a slurry.

The wet slurry 25 is then cast into mold 26, and the slurry is pressedunder hydraulic press 27, dewatering the slurry and producing a greencasting 28. The pressure employed determines the density of the finalproduct, densities in the range of about 40 lbs./ft³ to about 75lbs./ft³ being readily attained. For most applications a density of atleast about 60 lbs./ft³ is preferred.

The green casting is then conveyed into oven 29, where the gypsum isset, and the set casting is dried. If desired, dried set casting 30 maybe sanded at sanding station 31 to the desired thickness and/or then cutto the desired size at saw 32. It will be evident the aforesaid stepscan be adapted to either a batch or continuous process.

Another preferred process for manufacturing product within the scope ofthe present invention is a continuous process in which theaforementioned aqueous dispersion of constituents is formed into a sheetof indefinite length by use of standard paper-making techniques. Forexample, the aqueous dispersion of constituents can be fed from a headbox of the type associated with a paper-making machine to a foraminousmoving belt through which water drains as the mass of solids coagulatesand sets. The resulting composite sheet is consolidated by passingthrough press rolls. Heated rollers can be used to dry the sheet.

The process of the present invention can be used to make an unsupported(unfaced) product which has a substantially uniform and homogeneouscomposition throughout its thickness. The term "unfaced" is used hereinto mean that the product is not faced with a sheet material, forexample, of the type that is used as a facing material for gypsumwallboard--paper and glass fiber mat being examples of such facingmaterials. It should be understood, however, that the product of thepresent invention can be faced with such materials, if desired.

As mentioned above, it is preferred that the building material of thepresent invention have a density of at least about 60 lbs./cu. ft., butthat the material can have a density of as low as 40 lbs./cu. ft. Inorder to achieve flexural strength and screw-holding capacity having thevalues referred to above (40 lbs. and 400 lbs. respectively) in buildingmaterials having densities below 60 lbs. (cu. ft., there should beincluded in the composition from which the building material is maderelatively high amounts of binder polymer, for example, about 25 toabout 35 wt. % based on solids content. For applications where suchflexural strength characteristics and screw-holding capacity are notconsidered important, the use of such binder polymer can be in smalleramounts or avoided. Ceiling tiles are an example of such an application.Density of the building material can be controlled by use of pressure informing the product and/or by use of a low-density material, forexample, expanded perlite.

Table I presents data obtained in producing building materials using theaforesaid process and various cellulosic fibers. All samples werepressed at 300 psi in a 4"×4" mold. Samples were removed from the moldafter pressing, hydrated, and dried at 110° F. The samples were thensanded to about 0.3" thick, cut to 1" wide×4" long and tested forflexural on 3" centers. As reported in Table I below, the variouscellulosic fibers used in forming the building materials comprise eitherwood chips which are pulped (comparative example) or paper stock whichis pulped, the paper stock comprising either newspaper, Kraft paper,sulfite paper or paper of the type used to face wallboard. Thesignificant improvements achieved by using the pulp of paper stockrelative to the use of wood pulp are evident from the results reportedin Table I.

                                      TABLE I                                     __________________________________________________________________________    FIBERBOARD SAMPLE DATA                                                                       Cellulosic Fiber                                                              Newspaper                                                                           Kraft                                                                             Wood Pulp                                                                           Sulfite                                                                           Wallboard                                  __________________________________________________________________________    Weight of Water (g)                                                                          500   500 500   500 500                                        Weight of fiber material (g)                                                                 20    20  20    20  20                                         Time to vortex (sec)                                                                         24    291 0     191 377                                        Weight of Pulp Solution (g)                                                                  508.9 505.6                                                                             512.3 512.7                                                                             511.3                                      Weight of Gypsum (g)                                                                         113   113 113   113 113                                        Weight in mold prior to press (g)                                                            616   611.7                                                                             620.3 618.2                                                                             614.5                                      Water/gypsum off before press (g)                                                            267.5 265.7                                                                             383.9 273.2                                                                             289                                        Water/gypsum off during press (g)                                                            159.7 158.7                                                                             98.7  142.6                                                                             137.3                                      Gypsum lost before press (g)                                                                 2.82  5.47                                                                              36.59 5.07                                                                              1.33                                       Gypsum lost during press (g)                                                                 0.61  0.73                                                                              4.02  0.74                                                                              0.3                                        Total water lost during press (g)                                                            423.77                                                                              418.2                                                                             441.99                                                                              409.99                                                                            424.67                                     Weight of sample-wet (g)                                                                     180.98                                                                              179.7                                                                             126.7 177.28                                                                            183.12                                     Weight of sample-dry (g)                                                                     137.74                                                                              134.41                                                                            84.78 134.71                                                                            139.52                                     Water lost during drying (g)                                                                 43.24 45.29                                                                             41.92 42.57                                                                             43.6                                       % mixture actually in sample                                                                 97.3  96.6                                                                              98    97.7                                                                              97.1                                       % total water lost before drying                                                             87.8  87.9                                                                              98.5  85.1                                                                              87.8                                       % total gypsum lost before drying                                                            0.031 0.057                                                                             36.7  0.053                                                                             0.015                                      Weight of 1" sample (g)                                                                      25.4  24.92                                                                             15.59 26.66                                                                             24.52                                      Caliper of 1" sample (in)                                                                    0.318 0.321                                                                             0.314 0.322                                                                             0.319                                      Density of 1" sample (lb/in.sup.3)                                                           73.187                                                                              71.731                                                                            46.352                                                                              76.52                                                                             70.498                                     Flexural strength (lbs)                                                                      40.44 45.3                                                                              1.05  50.23                                                                             29.67                                      __________________________________________________________________________

In contrast to that taught in U.S. Pat. No. 4,557,973, it is notnecessary in the process described above to pretreat the glass wool withpowdered gypsum prior to its use. Furthermore, the presence of the vastexcess amount of water permits the gypsum slurry to flow out in the moldto a uniform thickness. In pressing the slurry to produce the greencasting, care is required in order to prevent geysering, in whichstreams of slurry suddenly exit the mold with a great deal of force.This can be avoided by applying pressure slowly to the slurry. Theamount of polymer binder in the slurry has an effect on the press timeas shown in FIG. 2. The data shown in FIG. 2 were obtained from castingcompositions of this invention prepared as set forth in Example 1.

EXAMPLE 1

Four casting compositions were prepared containing the followingingredients in parts by weight:

    ______________________________________                                                   Compositions                                                       Ingredients  A       B        C      D                                        ______________________________________                                        gypsum hemihydrate                                                                         113     113      113    113                                      newspaper     20      20       20     20                                      polymer binder.sup.a                                                                        0         2.2      4.4    8.7                                   water        500     500      500    500                                      ______________________________________                                         .sup.a Polyvinyl acetate (UCAR 130)                                      

The newspaper was pulped in a Waring blender; the gypsum and polymerbinder were added, and the blended mixture was pressed into 3/4 in.thick slabs at a rate to avoid geysering.

Building materials within the scope of this invention, prepared by theprocess described hereinabove, were tested for flexural strength. Thesetests generally employed ASTM Method C 473-86a modified in that thespecimens were 1/2 inch thick, 1 inch wide, and 4 inches long, withrandom orientation. In each case, the specimen was supported on bearings3 inches apart, and the specimen was broken across the 1 inch width.Evaluation of flexural strength as a function of the density of somebuilding materials was undertaken, and the results appear in FIG. 3. Theflexural strength of building materials within the scope of thisinvention generally should be at least about 40 lb., preferably at leastabout 60 lbs. (1/2" thick sample).

Building materials within the scope of this invention were evaluated forscrew-holding capacity. In these tests, a specimen of material to betested, dried to constant weight and 1/2"+/-1/32" thick, at least 9"long, and nominally 19/16" wide, was employed. At midwidth, a 5/32"pilot hole was drilled to receive a No. 12 sheet metal screw. With thespecimen supported on a wooden block or steel plate and the pilot holecentered over a 5/8" hole in the support, the screw was turned until thefull shank thickness penetrated the specimen. Force was then appliedvertically at the center of the screw, forcing the screw through thespecimen, and the force was noted. Evaluation of the screw-holdingcapacity of building materials within the scope of this invention wasundertaken, as set forth in Example 2. The results appear in FIG. 4. Ingeneral, the screw-holding capacity of a building material within thescope of this invention should be at least about 400 lbs., and in abuilding board to be used as fire door edge banding, the screw-holdingcapacity should be at least about 700 lbs.

EXAMPLE 2

A casting composition was prepared containing the following ingredientsin parts by weight:

    ______________________________________                                        Ingredient       Quantity                                                     ______________________________________                                        gypsum hemihydrate                                                                             113                                                          newspaper         20                                                          water            500                                                          ______________________________________                                    

The newspaper was pulped in a Waring blender; the gypsum was added, andvarying amounts of the blended mixture were added to a mold and pressedto 1/2 in. thick slabs. After curing and drying, the density andscrew-holding capacity of each slab was measured.

The screw-holding capacity of the building materials of this inventionis enhanced through the use of binder polymer as illustrated in Example3.

EXAMPLE 3

In each case, the casting composition included 113 g calcined nonfibrousgypsum, 20 g paper, 2.2 g. glass textile fiber, and 5 g binder polymer.

    ______________________________________                                                     Product Density                                                                           Screw-Holding Cap'y.                                 Binder Polymer                                                                             lbs./ft.sup.3                                                                             lb.                                                  ______________________________________                                        AIRFLEX                                                                              4530.sup.a                                                                              67.5        749                                                     4500      65.8        670                                                     7522      68.6        730                                                     4514      68.8        689                                              UCAR 130.sup.b                                                                             66.6        890                                                  GEN FLO 6500.sup.c                                                                         68.9        718                                                  UCAR 376.sup.d                                                                             71.6        678                                                  UCAR 417.sup.e                                                                             66.7        650                                                  ______________________________________                                         .sup.a The AIRFLEX products are all ethylene/vinyl chloride                   .sup.b Polyvinyl acetate homopolymer                                          .sup.c Styrene/butadiene                                                      .sup.d Vinyl/acrylic                                                          .sup.e Acrylic                                                           

With reference now to FIGS. 5 and 6, a solid fire door 40, nominally 4ft. wide and 8 ft. high, was constructed employing the core material 43described in U.S. Pat. No. 4,159,302, which is incorporated herein byreference. The building board of this invention was utilized, not onlyin the edge banding 45, but also as blocking 44, to provide support forthe latch mechanism, etc. The door facings 42 were made of birch veneer,1/8" thick, and natural fir strips 41 were adhesively bonded to thevertical edge banding (stiles). The solid fire door 40 was tested inaccordance with ASTM E-152, and the door achieved a 1.5 hr. fireendurance rating.

With reference now to FIGS. 7 and 8, a fire door having a partiallyhollow core was constructed along the lines of that shown in FIG. 5 ofU.S. Pat. No. 4,811,538, the disclosure of which is incorporated hereinby reference. The approximately 4 ft. wide and 8 ft. high door 60differed, however, from the aforementioned patented door in that it didnot contain a central gypsum core material of the type described in U.S.Pat. No. 4,811,538. The door 60 comprises natural fir edge banding 65and facings 62 which are spaced with cardboard honeycomb 66, the wholeassembly being adhesively secured together. Facings 62 were made fromapproximately 1/8 in. thick building panel material within the scope ofthis invention and having the following composition by weight:

    ______________________________________                                        gypsum dihydrate   83.9%                                                      newspaper          12.7%                                                      glass textile fiber (1/2 in.)                                                                    0.4%                                                       glass wool         0.5%                                                       poly(vinyl acetate).sup.a                                                                        2.5%                                                       ______________________________________                                         .sup.a UCAR 130                                                          

The door so constructed was tested according to the method of ASTM E 152and achieved a 20 min. fire endurance rating.

It will be evident that the compositions within the scope of thisinvention can be employed to make any number of different structuralbuilding materials, all within the scope of this invention, but notspecifically enumerated hereinabove. Although several specific buildingproducts utilizing this invention have been named, many more will beevident to those skilled in the art. Such products include, for example,underlayment for plastic or ceramic counter tops; floor underlayment forceramic tile, plastic or other floor covering; fire resistant roofsheathing (plywood); fire resistant wall panels; fire resistant panelsfor around furnaces, fireplaces, stoves and other heat emittingappliances; backer for wood or plastic veneer in furnituremanufacturing; cross banding for fire rated doors; door skins for firerated doors; fire resistant boards for safe liners; and fire resistantpanels for lining elevator shafts.

What is claimed is:
 1. A method of preparing a fire door capable ofachieving a 20 minute ASTM E-152 fire test rating without the use ofasbestos, comprising:(a) providing at least a pair of substantiallyasbestos free preformed gypsum fiberboard panels having a compositioncomprising about 65 wt. % to about 90 wt. % set gypsum dihydrate, about10 wt. % to about 17 wt. % paper fiber, and a density of at least about60 lbs/cu.ft., said fiberboard panels being formed by casting andcompressing a wet slurry comprising gypsum and said paper fiber; (b)providing a core; (c) adhering said core between said gypsum fiberboardpanels; (d) securing edge banding between said gypsum fiberboard panelsto substantially enclose said core and form a composite member, saidedge banding comprising about 65 wt. % to about 90 wt. % set gypsumdihydrate and from about 7 wt. % to about 30 wt. % paper fiber, saidedge banding having a density of at least about 60 lbs/cu.ft. and ascrew holding capacity of at least 400 pounds: and (e) providing saidcomposite member as a substantial portion of said door so as to producea fire door capable of achieving a 20 minute ASTM E-152 fire testrating.
 2. The method of claim 1, wherein said core comprises ahoneycomb structure.
 3. The method of claim 1, wherein said core issolid.
 4. The method of claim 1, further comprising disposing a veneerhaving the appearance of wood on a front and rear substantially planarexposed surface of said composite member.
 5. The method of claim 1,wherein said edge banding comprises about 68 wt. % to about 78 wt. % setgypsum dihydrate, about 10 wt. % to about 17 wt. % paper fiber, up toabout 15 wt. % binder polymer, up to about 7 wt. % inorganic fiber, upto about 6 wt. % clay, and up to about 6 wt. % vermiculite, said edgebanding having a density of at least 60 lbs/cu.ft. and a screw-holdingcapacity of at least about 400 lbs.
 6. The method of claim 5, whereinsaid inorganic fiber of said fiber board panels comprises siliceousfiber in an amount from about 0.8% to about 2% based on the weight ofsaid panels.
 7. The method of claim 5, wherein said edge banding has ascrew-holding capacity of at least about 700 lbs.
 8. The method of claim1 wherein said fire door is capable of achieving a 1.5 hour ASTM E-152fire test rating.
 9. A method of preparing a fire door capable ofachieving a 20 minute ASTM E-152 fire test rating without the use ofasbestos, comprising:(a) providing at least a pair of substantiallyasbestos free preformed gypsum fiberboard panels having a compositioncomprising about 65 wt. % to about 90 wt.% set gypsum dihydrate, about10 wt. % to about 17 wt. % paper fiber, and a density of at least about60 lbs/cu.ft., said fiberboard panels being formed by casting andcompressing a wet slurry comprising gypsum and said paper fiber; (b)providing a core; (c) adhering said core between said gypsum fiberboardpanels; (d) securing edge banding between said gypsum fiberboard panelsto substantially enclose said core and form a composite member, saidedge banding comprising about 65 wt. % to about 90 wt. % set gypsumdihydrate and from about 10 wt. % to about 17 wt. % paper fiber, saidedge banding having a density of at least about 60 lbs/cu.ft. and ascrew holding capacity of at least 400 pounds; and (e) utilizing saidcomposite member as a substantial portion of the door to produce a firedoor capable of achieving a 20 minute ASTM E-152 fire test rating. 10.The method of claim 9, wherein said core comprises a honeycombstructure.
 11. The method of claim 9 further comprising disposing aveneer having the appearance of wood on a front and rear substantiallyplanar exposed surface of said composite member.
 12. The method of claim9, wherein said edge banding comprises about 68 wt. % to about 78 wt. %set gypsum dihydrate, about 10 wt % to about 17 wt. % paper fiber, up toabout 15 wt. % binder polymer, up to about 7 wt. % inorganic fiber, upto about 6 wt. % clay, and up to about 6 wt. % vermiculite, said edgebanding having a density of at least about 60 lbs/cu.ft. and ascrew-holding capacity of at least about 400 lbs.
 13. The method ofclaim 12 wherein said inorganic fiber of said fiber board panelscomprises siliceous fiber in an amount of from about 0.8% to about 2%based on the weight of said panels.
 14. The method of claim 12, whereinsaid edge banding has a screw-holding capacity of at least about 700lbs.
 15. The method of claim 9 wherein said fire door is capable ofachieving a 1.5 hour ASTM E-152 fire test rating.
 16. A method ofpreparing a fire door capable of achieving a 20 minute ASTM E-152 firetest rating without the use of asbestos, comprising:(a) providing atleast a pair of substantially asbestos free preformed gypsum fiberboardpanels having a composition comprising about 65 wt. % to about 90 wt. %set gypsum dihydrate, about 7 wt. % to about 30 wt. % paper fiber, adensity of at least about 40 lbs/cu.ft., said fiberboard panels beingformed by casting and compressing a wet slurry comprising gypsum andsaid paper fiber, and binder polymer; (b) providing a core; (c) adheringsaid core between said gypsum fiberboard panels; (d) securing edgebanding between said gypsum fiberboard panels to substantially enclosesaid core and form a composite member, said edge banding comprisingabout 65 wt. % to about 90 wt. % set gypsum dihydrate and from about 7wt. % to about 30 wt. % paper fiber, said edge banding having a densityof at least about 60 lbs/cu.ft. and a screw holding capacity of at least400 pounds; and (e) utilizing said composite member as a substantialportion of the door to produce a fire door capable of achieving a 20minute ASTM E-152 fire test rating.
 17. The method of claim 16 whereinsaid core comprises a honeycomb structure.
 18. The method of claim 16further comprising disposing a veneer having the appearance of wood on afront and rear substantially planar exposed surface of said compositemember.
 19. The method of claim 16 wherein said edge banding comprisesabout 68 wt. % to about 78 wt. % set gypsum dihydrate, about 10 wt % toabout 17 wt. % paper fiber, up to about 15 wt. % binder polymer, up toabout 7 wt. % inorganic fiber, up to about 6 wt. % clay, and up to about6 wt. % vermiculite, said edge banding having a density of at leastabout 60 lbs/cu.ft. and a screw-holding capacity of at least about 400lbs.
 20. The method of claim 19 wherein said inorganic fiber of saidfiber board panels comprises siliceous fiber in an amount of from about0.8% to about 2% based on the weight of said panels.
 21. The method ofclaim 16 herein said fire door is capable of achieving a 1.5 hour ASTME-152 fire test rating.